Supercritical fluids have received considerable attention as extraction solvents primarily owing to their desirable mass transfer characteristics Hawthorne, S. B., Anal. Chem. 62:633A-642A(1990). Carbon dioxide has received particular interest as a replacement solvent in extraction McHugh, M. A. and Krukonis, V. J., Supercritical Fluid Extraction: Principles and Practice, 2.sup.nd ed., Butterworth-Heinemann, Newton, Mass. (1994). Carbon dioxide is nontoxic, nonflammable, relatively inert, relatively inexpensive on a per extraction basis, and generally environmentally innocuous as compared with typical, often toxic, organic solvents.
One area of interest for the application of carbon dioxide is the reduction of contamination from processes involving metals which is a major environmental challenge. A first aspect of this area involves the remediation of legacy waste discharged into the environment and contained in storage. A second aspect of this area involves developing new environmentally friendly processes upstream which eliminate metal contamination sources. Typical metal solvent extraction techniques used for both remediation and processing involve the generation of additional solvent waste streams. Herein lies an opportunity to affect both ongoing processes involving metals and decontamination methods by taking advantage of carbon dioxide as an ideal replacement solvent.
Carbon dioxide is, however, a nonpolar solvent and consequently not capable of dissolving highly polar species such as metal ions. In traditional solvent extraction systems, chelating agents are utilized to bind the metal ions forming neutral metal complexes capable of dissolving in nonpolar organic solvents. As a nontraditional solvent, carbon dioxide poses its own difficulties with regard to appropriate chelating agents to achieve solubility of metal species. The vast majority of the work in this area has focused on utilizing conventional solvent extraction chelating agents such as dithiocarbamates, .beta.-diketones, crown ethers, and organophosphorous compounds U.S. Pat. No. 5,087,370, U.S. Pat. No. 5,221,480, U.S. Pat. No. 5,356,538, U.S. Pat. No. 5,770,085, U.S. Pat. No. 5,730,874, U.S. Pat. No. 5,606,724, U.S. Pat. No. 5,561,066, Saito et al, Bull. Chem. Soc. Jpn. 63:1532-1534 (1990), and Smart et al, Ind. Eng. Chem. Res. 36:1819-1826 (1997). Typically a large excess of the chelating ligand or a cosolvent is required. UO.sub.2 (NO.sub.3).sub.2.2TBP appears to be an isolated example of one particularly highly CO.sub.2 -soluble complex Carrott, M. J. et al Chem. Commun. 373-374 (1998). When TBP was used in a more general study for the extraction of lanthanides, greater than 10% TBP in CO.sub.2 as a solvent modifier was necessary to effect extractions higher than 25% Laintz, K. E. et al, Anal. Chem. 66:2190-2193. Fluorinated analogs of dithiocarbamate and .beta.-diketone chelating agents have shown enhanced solubility in CO.sub.2 over their hydrocarbon counterparts although a large excess of ligand is still necessary to improve extraction efficiency. This enhanced solubility is likely related to the enhanced volatility of metal complexes with fluorinated ligands. Lagalante, A. F. et al Inorg. Chem. 34:5781-5785 (1995). In some instances liquid CO.sub.2 may serve as a more attractive medium due to lower operating pressures.
Several studies have addressed the performance of chelating agents in liquid carbon dioxide extraction of metals. Laintz et al Anal. Chem. 70:440-404 (1998) found that liquid and supercritical carbon dioxide at the same density have comparable efficiencies for the extraction of nickel and zinc from spent plating bath solutions using acetylacetone and its fluorinated analog. Yadzi and Beckman U.S. Pat. No. 5,641,887 have developed a number of picolylamine, dithiol, and dithiocarbamate derivatives of Krytox acids that are soluble in liquid carbon dioxide. Similarly derivatized polysiloxanes were found to be much less soluble in CO.sub.2 than the fluorinated polyether Krytox chelates.
Clearly, both liquid and supercritical carbon dioxide have desirable qualities as extraction solvents. Therefore, the need for chelating agents useful for carbon dioxide systems, especially for relatively low pressure operations, is evident.